The present invention relates to a fuel control device for gas turbines and particularly to a fuel control device for a gas turbine for use with an electric generator which device controls the amount of intake fuel supplied to the gas turbine so as to maintain constant the revolution speed thereof.
In a double-axle gas turbine, including a compressor turbine and an output turbine, which drives an electric generator, fluctuations of the revolution speed of the output turbine appear directly as fluctuations of the electric current generated. Thus big fluctuations of the output revolution speed are not permissible.
In the near steady-state operation of the gas turbine, it is possible to control a fuel control servo valve on the basis of conventional feedback control (proportional and integral control) so as to maintain constant the revolution speed of the gas turbine. However, if a sharp fluctuation of the load takes place, a large lag in response in the fuel supply will occur, whereby the revolution speed of the output turbine will fluctuate beyond its permissible range. Thus, in the prior art, the flow rate of fuel to the turbine has been further controlled by an override fuel control system according to the load on the output turbine in such a way that, if the load sharply changes, the amount of fuel supplied will be changed by a corresponding amount. For example, when the load sharply decreases, a minimum flow rate of fuel substantially necessary to maintain the combustion is maintained and the remaining fuel supply is stopped. On the other hand, when the load sharply increases, an additional amount of fuel is supplied by a system different from the fuel control valve.
The specific circuit for this prior art control will be described, referring to FIGS. 1 and 2 of the accompanying drawings. A load sensor 10 senses the load on the output turbine and outputs a signal S10 proportional in magnitude to the load. This signal is differentiated with respect to time in a differentiating circuit 12 to obtain a signal S12 corresponding to the rate of change of the load. The signal S12 is compared with reference values S14R and S16R in a fuel increase comparator 14 and in a fuel decrease comparator 16, respectively. The fuel increase comparator 14 outputs a signal S14 of high level "1" while S12 is larger than S14R so as to operate a drive circuit 18 for a sharp fuel increase valve (not shown), thereby temporarily increasing the flow rate of fuel according to a sharp increase in the load.
Similarly the decrease comparator 16 outputs a signal S16 of high level "1" while S12 is smaller than S16R so as to operate a drive circuit 20 for a sharp fuel decrease valve such as a relief valve (not shown), thereby temporarily decreasing the flow rate of fuel according to a sharp decrease in the load. This system is of course fitted in addition to the normal feed back fuel control system.
However, in spite of such a control system for fuel supply, repetitions of sharp load fluctuations in a short time will cause the following problem. For example, when the load sharply decreases and immediately thereafter sharply increases, the fuel supply to, and therefore the revolution speed of, the compressor turbine decreases and immediately thereafter the fuel supply increases. This surge in the fuel supply to the turbine largely shortens the life time of the turbine. Further, as a matter of course, immediately after the sharp decrease in the load and the fuel supply, the revolution speed of the output turbine is likely to increase rather above the setting point, due to the sharp decrease in the load. Accordingly the subsequent sharp increase in the load does not require so sharp a corresponding increase in the fuel supply as it would require, if not preceded by a sharp decrease in the load.